Corona treatment of plastic film has long been used in the production of merchandise bags. This application of high voltage electricity to the surface of the film alters the bond structure of the film material and increases the relative tackiness of the film over untreated film. The increase in tackiness is desirable in that it increases the adhesion of ink to the film. The increase in tackiness is also desirable in that it increases the adhesion of one bag surface to another. This permits the development of self-opening bags in which the rear surface of one bag adheres to the front surface of a subsequent bag, allowing the first bag to pull the subsequent bag open upon dispensing of the first bag. Many variations upon the methods of corona treating of plastic and other film materials have been developed as shown in the following references.
U.S. Pat. No. 5,469,969, issued to Huang, discloses a self-opening plastic bag pack system. The self-opening feature of the bags described and manufactured in accordance with the method in this patent depends upon a central tab portion, wherein the central tab portions are heat welded together. The manufacturing process involves the use of a cold and somewhat thick and dull die blade to form the tearing cut and thus the pack of bags are cold pressed together to create frangible pressure bonding of the rear wall of handles of the top bag to the forward wall of the handles of the next bag. This cold pressing process may be different than the localized pressure that is used to create adhesion normally found in the prior art. The individual bags are formed from a continuous tube of plastic material which is treated on its entire outside surface (both top and bottom or front and back) by corona surface treatment, wherein the plastic material is passed over guide rollers and treatment rollers which are grounded. Discharge electrode bars are spaced slightly over the plastic material to surface treat the entire surface of the plastic material. Electrode bars are located both above and below, grounded in treatment rollers.
U.S. Pat. No. 5,484,376, issued to Prader is directed to a ready to load bag pack and the method of forming said system. In order to obtain a bag pack with satisfactory self-opening properties, wherein the second bag in the stack is opened at the mouth when the first bag is removed from the stack, it has been found that the desirable property may be achieved by placing corona discharge treated polyethylene film surfaces together and this is achieved by treating the complete outside surface as opposed to only one surface (top or bottom). The schematic representation of the method used in this invention is seen in FIG. 3 wherein the film of thermoplastic material is passed between two oppositely disposed corona discharge treaters which are positioned to treat outside surfaces (both top and bottom) of the collapsed tube. The region may be uniformly treated or may be treated in interrupted areas such as in dots, squares, or stripes in order to achieve the necessary adhesion so that the mouth region of a next succeeding bag will be opened to a meaningful extent upon removal of a first bag.
U.S. Pat. No. 3,391,070, issued to Morgan, is directed to a method of selectively treating a plastic film to improve anchorage characteristics is another example of a plastic film being treated on both sides of a collapsed tube by means of first treater bar and a second treater bar located on opposing sides of two grounded backup rollers.
U.S. Pat. No. 3,509,859, issued to Concarovs et al. is directed to corona discharge treatment of polymer film to increase the adhesion characteristics thereof. This patent provides another example of a plastic web being treated on both outside surfaces as it passes under and over a pair of grounded rollers and treated by a pair of electrodes in order to achieve the corona discharge treatment on both surfaces of the web.
U.S. Pat. No. 6,153,059, issued to Wadsworth et al. is directed to a composite of pleated and non-woven webs and the method and apparatus for the electrostatic charging of same. A web of thermoplastic material is charged on both sides by means of a pair of charging bars opposed by a pair of charging drums wherein the web is charged on the two sides to different polarities.
U.S. Pat. No. 5,669,504, issued to Leone et al. is directed to a thermoplastic bag closure. It includes the processing of a film material by two oppositely disposed corona discharge treaters positioned so as to treat the outside surfaces of the collapsed tubes.
U.S. Pat. No. 6,942,100, issued to Su discloses a square bottomed plastic bag stack and method of making the same. Among the features seen for the plastic bag packs described in this patent, includes the means for attaching the upper portion of the rear wall of a leading bag to an upper portion of a front wall of a subsequent bag in the stack. Thus, when the leading bag is pulled from the stack, the subsequent bag will cause the leading bag to open. The means for attaching the upper portion of the rear wall of a leading bag to the upper portion of the front wall of a subsequent bag may be selected from the group that includes corona treatment alone, as opposed to pressure alone and/or corona treatment with pressure.
U.S. Pat. No. 5,690,229, issued to Piraneo et al. discloses an easy dispense T-shirt bag. This patent addresses the issue of preparing an easy-opening plastic bag system by controlling the power of the corona discharge so that the interior surfaces of the bags are not welded together. Furthermore, the pressure welding involved in the formation of these bags, requires no additional steps in the manufacturing process, and furthermore, a static charge may be induced on the bag so that each bag is at a different voltage, thus resulting in individual bags attracting one another while the panels of the individual bags repel one another.
It is an objective of the present invention to provide a method and apparatus for corona treating bag film material so as to provide improved tackiness of the material. It is a further objective to provide such an improvement in surface tackiness without a decrease in film treating speed. It is a still further objective of the invention to provide the improvement without requiring greater amounts of electric power. It is yet a further objective to provide a sufficient increase in tackiness to allow for the construction of self opening bags without the need for localized pressure on a bag stack. Finally, it is an objective of the present invention to provide such benefits with relatively minor changes in treating equipment requirements.
While some of the objectives of the present invention are disclosed in the prior art, none of the inventions found include all of the requirements identified.